This invention relates to methods and systems for concrete fabrication. More particularly, this invention relates to methods and systems for reclaiming concrete mix returned to a ready-mix plant by concrete trucks in the course of a day's production.
Concrete batch fabrication plants are well known and typically consist of a source of plant water and storage bins for storing fresh cement powder, and aggregates of various sizes to be admixed to the cement powder and water when a batch of concrete is required. The mixing operation is typically done either manually or automatically in a plant termed a "ready-mix" plant under the control of an operator or a computerized batching console by selecting the proportions of the various constituents to be admixed. In the latter type of installation, the batching console is monitored by a plant operator. Fresh batched concrete mix is dispensed into one or more waiting trucks each having a revolving drum which is used to transport the fresh batched concrete mix to one or more job sites for pouring. After a pour has been made, each truck typically returns to the concrete fabrication yard where a fresh batch of concrete is poured into the truck mixing drum for transportation to the job site. This cycle continues throughout the production day, or until no more fresh concrete mix is required at any of the job sites. At the end of a production day, each truck is finally washed out to remove the accumulated deposits within the mixing drum and parked until the next production day.
For years, the concrete remaining in a returning truck was simply dumped in the yard, permitted to harden and later hauled away for use as land fill. However, this arrangement was found to be extremely wasteful in that reusable aggregate constituents of the returned concrete were thrown away. In addition, the dumping of returned concrete in a yard was found to be environmentally unsound. Accordingly, installations were designed for the purpose of attempting to reclaim portions of the returned cement mix and to provide a less environmentally damaging operation. Such installations employed large drag tanks into the inlet end of which the returned concrete is dumped from a returning truck, the drag tanks having continuously operated drag chains and separate sedimentation chambers for assisting in the separation of the returned concrete constituents and clarification of the water portion thereof. While representing an improvement, drag tank installations were found to require relatively large amounts of electrical power to operate, were found to suffer from mechanical bearing failures due to the heavy loads involved, were expensive to install and maintain, and produced unsightly residue piles of fine sand and "spent" cement residue which eventually had to be hauled away to a fill site.
As an improvement over drag tank installations, slurry recirculation installations were designed to provide aggregate and cement slurry reclamation with less environmentally contaminating effect. In such installations, the returned aggregates are separated by a screw classifier from the remaining constituents (the slurry), sized and stored for later reuse; while the slurry is recirculated through large storage tanks, or continuously agitated in large above ground storage tanks, diluted with fresh water from the plant site, and added back to some of the freshly batched concrete mix. Such systems, while less environmentally damaging than the drag tank type installation, suffer from the disadvantages of requiring exceedingly large storage silos for the slurry, continuous consumption of large amounts of electrical power in order to recirculate the slurry, and extensive cost in erecting the silos and the associated plumbing. In addition, since the age of the slurry cannot be accurately determined, the effect on the freshly batched concrete mix of adding the slurry cannot always be predicted with certainty, and thus the structural characteristics possessed by the hybrid concrete mix cannot always be determined so that such hybrid concrete mix may only be legitimately used for limited, predominantly non-structural, purposes.